This invention relates generally to the recovery of materials from refuse, and specifically to a process for treating refuse to obtain fuel therefrom.
Solid waste materials of the type typified by municipal waste have traditionally presented problems of disposal which have become increasingly critical in recent years as a result of not only a rapidly increasing population but the compounding difficulty of a drastic increase in per capita production of solid waste. Conventionally, such solid waste has been disposed of by such means as incineration and landfill. Obviously with the ever increasing concern with problems of natural resources and the dwindling supply of acreage suitable for landfill operations within a reasonable distance of population centers, both of these methods of solid waste disposal have become less acceptable.
Proposals have been been made from time to time for processing solid waste materials in such manner that various components thereof can be reused, and processes and systems have been recently developed which have proved successful in recovering for reuse the mineral components, including glass, of municipal solid waste -- in accordance particularly with one or more of Marsh U.S. Pat. Nos. 3,720,380, 3,945,575 and 3,970,254. Success has also been achieved in the recovery for reuse of paper making fiber from municipal solid waste -- in accordance with Marsh U.S. Pat. No. 3,736,223. In every case, however, there remains a substantial volume of organic materials which must be disposed of, and the primary treatments for disposal are landfill and burning.
Landfill operations are usually complicated by problems of both distance, as noted above, and especially bulk. This tends to make burning the preferred alternate, and since burning necessarily results in the generation of heat, efforts have been made to use municipal waste as a source of fuel, as contrasted to merely incinerating it, because it contains a large amount of combustible materials. Generally, such efforts proved troublesome because the energy output of the waste was inconsistent due to lack of uniformity of the fuel content of the waste materials and the presence of incombustibles such as inorganic materials and the like. The lack of uniformity of the fuel content was due to lack of adequate mixing of the materials, and resulted in wide fluctuations of the heat content, low efficiency, and high ash.
Efforts have been made to concentrate the combustibles by processing the waste material prior to its use as fuel, such as by shredding and air-classifying the municipal waste to sort out the heavy inorganic materials and retain the lighter combustibles. But the lack of uniformity of the combustibles content remained as a problem in terms of consistent energy output because the waste could not be mixed and still be able to be air classified. Also, the separation step has the consequence that a certain amount of the combustible material is wasted. Further, the inorganic materials were subsequently disposed of by dumping and landfilling.
Marsh U.S. Pat. No. 3,830,636 teaches that the consistency and uniformity of the recovered combustible organic materials are substantially improved by wet processing the entire mass of solid waste materials, and such treatment also promotes the recovery of the inorganic materials. The wet processing reduces the size of the combustible organic materials and allows for better mixed and concentrated fuel. Also, the wet processing is gentler to inorganics such as glass and promotes their recovery. But the fuel recovered by a wet process contains a relatively large amount of moisture, and while it is practicable to reduce the moisture by mechanical pressing to about 50%, reduction below that amount generally requires additional energy consuming processing steps, such as drying, which decreases the overall thermal value of the fuel.